Paper, cardboard, and many other products are formed using pulp fibers, such as wood, straw, bamboo, or seed fibers. Wood fibers and other vascular plant fibers typically contain cellulose, hemicellulose, extractives, and lignin. Extractives typically include substances such as resin acids, fatty acids, turpenoid compounds, and alcohols. A bleaching process is typically used to remove at least some of the lignin and extractives from these fibers. The amount of lignin and extractives removed typically varies with the type of product being formed. For example, writing paper typically contains very little lignin, while cardboard typically contains more lignin.
The effectiveness of a bleaching process is traditionally determined using brightness measurements as defined by the International Organization for Standardization (ISO). The brightness measurements often involve measuring the reflectance of a finished product at a central wavelength of 457 nanometers (in the visible light spectrum).